There are many machines which include a traction vehicle and one or more "working units" attached thereto. For example, crop harvesting machines typically include large wheeled vehicles supporting one or more vegetation cutters. Although the present invention can be applied to a wide variety of machines having traction vehicles and working units, one particularly advantageous application is a turf maintenance machine such as a turf mower having one or more turf cutting units. The cutting units of a turf mowing machine can be of the rotary, reel or flail type. Turf mowers having rotary cutting units or decks are often referred to as rotary mowers since the blades revolve in a rotary fashion within the cutting unit. The present invention will be described in terms of a riding (as opposed to a walk-behind) rotary mower having one or more rotary cutting units or decks, but those skilled in the art will recognize that the invention could be applied to a wide variety of harvesting and turf maintenance machines.
Larger rotary mowers typically include a traction vehicle supported by a plurality of wheels; a prime mower connected through a transmission to one or more of the wheels; one or more decks connected through a power take-off (PTO) to the prime mover; and one or more lift arms (or analogous structures) pivotally connected to the traction vehicle suitable for supporting the deck(s).
Preferred rotary mowers have several features. Certainly, the traction vehicle must indeed provide adequate traction under a wide variety of conditions. The vehicle must be able to climb hills, allow for sidehill cutting, and provide braking even in downhill and/or wet conditions. Further, the vehicle must provide enough traction so that turf mowing can be done quickly and efficiently. If the traction wheels slip when the vehicle encounters a hill, the operator must typically return to the bottom of the hill and reattempt an ascent. Once traction is lost, e.g., on a wet hill, and the drive wheels begin to slip, it is nearly impossible to reestablish traction.
Another important characteristic of rotary mowers is the ability of the decks to "float" relative to the traction vehicle. A "floating" cutting unit can follow the contours or undulations of the ground irrespective of the gross motion of the traction vehicle. The cutting units are normally supported by ground following devices, e.g., casters, skids or rollers, and the lift arm/traction vehicle joint and lift arm/deck joint preferably allow the decks to follow the intermediate ground countour. If cutting units are not supported in a floating manner, they tend to scalp the higher regions and miss the lower regions. While non-floating cutting units might be acceptable for residential use, certainly golf course and estate maintenance require that the cutting units individually follow the subtle variations of the ground to maintain a consistently good cut across the entire swath, regardless of the immediate topography encountered by each individual cutting unit.
To provide adequate traction, the traction vehicle itself should bear most of the weight of the mowing machine. On the other hand, to provide a consistent height of cut, irrespective of ground undulations, the cutting units should be self supporting to some degree to allow their ground contacting devices to continuously contact the ground.
Given these two somewhat inconsistent requirements, i.e., traction and ground hugging capability, it has long been understood that it is desirable to provide a degree of partial counterbalancing of cutting units in their working positions. That is, prior art cutting units have been supported such that a portion of their weight is borne by the traction vehicle. For example, reference is made to U.S. Pat. No. 4,307,559.
Traction is obviously most important on hills, whether uphill, sidehill or downhill. Operator safety is at stake, and wheel slippage can also force the operator to stop and start repeatedly. Further, wheel slippage also causes undesirable damage to the turf. In fact, hill traction is so important Applicants have discovered that most operators are willing to sacrifice some height of cut control to enhance hill traction. On level ground, traction is less important and it is therefore possible to allow the cutting units to be more self supporting to maximize their ground hugging capability. Therefore, Applicants perceive that it is desirable to have a first traction vehicle/cutting unit weight distribution on level ground (where relatively less weight on the traction wheels is necessary), and a second traction vehicle/cutting unit weight distribution on hills, where there is a need for a greater amount of weight on the traction wheels (where relatively more weight on the traction wheels is necessary). The horsepower required to propel a mowing machine uphill is most dependent upon the gross vehicle weight and is not primarily related to the weight carried by the drive tires. Therefore, traction can be improved by transferring some of the weight from the cutting units to the traction vehicle, without creating a greater load on the prime mover. Another advantage associated with transferring weight from the cutting units to the traction vehicle is an increase durability of the ground following devices, e.g., rollers, casters, or skids, which support the cutting units.
As noted above, the desirability of providing a degree of cutting unit counterbalance has long been recognized. It has also been recognized that it is desirable to provide an adjustable cutting unit counterbalance, so that more or less weight can be transferred to the traction vehicle drive wheels depending on conditions. Reference is made to the Groundsmaster.RTM. 220 unit, manufactured by The Toro Company, assignee herein, which includes springs adjustably supporting the lift arms of a mower. While springs are somewhat useful for deck counterbalancing because of their simplicity and cost effectiveness, they are large and cumbersome and tend to vibrate in the absence of extensive damping. Counterbalancing springs are not easily adjusted, particularly during mowing operations; are liable to fatigue failure; and of course a separate mechanism, typically a hydraulic mechanism, must be included to raise and lower the cutting unit. Also, the springs cannot be easily adjusted to provide variable counterbalancing depending on conditions.
Hydraulic counterbalancing has been suggested as a means for overcoming the disadvantages of a spring system. In such systems hydraulics are used to raise and lower the cutting units, and also to provide partial counterbalancing. For example, an adjustable hydraulic system such as the one shown in FIG. 1 has been made available. Referring to FIG. 1, the prior art system broadly includes a pump; a single-acting cylinder or ram; a multi-position selector valve system; and a manually-operated proportional valve. The hydraulic ram used to raise and lower the deck is also used to counterbalance the deck in its lower, or working, position. The multiple-position selector valve system is included to allow the operator to raise, lower, lock or float the deck, by appropriately controlling the flow of hydraulic fluid to and from the ram. The manually-operated proportional valve controls the pressure in the return line when the deck is being floated, and therefore establishes the hydraulic "back pressure" within the cylinder which determines to what degree the deck is floated. The operator can adjust the "float" valve as the mower is operating to gradually dial in a greater floating pressure (less weight on the cutting unit, more weight on the traction unit drive wheels) or a lesser floating pressure (more weight on the cutting unit, less weight on the traction unit drive wheels) depending on conditions. It should particularly be noted that the prior art system shown in FIG. 1 includes only a simple single-acting cylinder.
While the general concept is laudable, there are several problems associated with the prior art adjustable hydraulic counterbalance system shown in FIG. 1. One serious drawback is that the system is too slow. Upon encountering different conditions, e.g., wet grass or hills, the operator must crank the valve one way or the other. He may not be able to properly adjust the proportional valve in time to prevent wheel slippage. And, as noted above, once traction is lost, it is very difficult to reestablish. Another problem associated with the prior art system shown in FIG. 1 is that it is imprecise. The operator gradually adjusts the valve until the mower "feels right." While this reliance on feel is acceptable for experienced operators it is simply unworkable for those will less experience. Only a highly skilled operator can ever know when the proportional valve is in the proper position to provide the proper weight distribution. Still another problem is that the proportional valve system is too burdensome. Many operators will simply not use a counterbalancing system that requires them to continually adjust a valve depending on the terrain and wetness. Given the problems associated with the proportional valve system shown in FIG. 1, operators naturally tend to select a compromise position somewhere in between maximum and minimum traction, rather than continually adjusting the valve in a futile chase of the optimal weight distribution. Relying on the operator to manipulate a manually operated proportional valve can be problematical. An operator, if given a choice, would normally set the system on the highest weight transfer setting to eliminate tire slippage in nearly every situation, but this would result in poor ground following.
The present invention addresses the problems associated with prior art cutting unit counterbalancing systems. In particular, the present invention includes a condition accommodating hydraulic counterbalance system which is fast, precise, and very easy to use. A preferred counterbalance or weight distribution state between the traction vehicle and the cutting unit(s) can thereby be established on hilly or level ground, and traction and contour following can be optimized.